Re: [PATCH v2 1/4] KVM: vmx/pmu: Add MSR_ARCH_LBR_DEPTH emulation for Arch LBR

Sean Christopherson
2021-03-01T23:46:12Z

On Wed, Feb 03, 2021, Like Xu wrote:
> @@ -348,10 +352,26 @@ static bool intel_pmu_handle_lbr_msrs_access(struct kvm_vcpu *vcpu,
> return true;
> }
>
> +/*
> + * Check if the requested depth values is supported
> + * based on the bits [0:7] of the guest cpuid.1c.eax.
> + */
> +static bool arch_lbr_depth_is_valid(struct kvm_vcpu *vcpu, u64 depth)
> +{
> + struct kvm_cpuid_entry2 *best;
> +
> + best = kvm_find_cpuid_entry(vcpu, 0x1c, 0);
> + if (depth && best)

> + return (best->eax & 0xff) & (1ULL << (depth / 8 - 1));

I believe this will genereate undefined behavior if depth > 64. Or if depth < 8.
And I believe this check also needs to enforce that depth is a multiple of 8.

For each bit n set in this field, the IA32_LBR_DEPTH.DEPTH value 8*(n+1) is
supported.

Thus it's impossible for 0-7, 9-15, etc... to be legal depths.

> +
> + return false;
> +}
> +